1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, which includes a step of annealing a semiconductor film by using a laser beam. Incidentally, the semiconductor device here indicates any devices capable of functioning by using semiconductor characteristics, and also includes an electro-optic device, such as a liquid crystal display device or a light-emitting device, and an electronic device including the electro-optic device as a part thereof.
2. Description of the Related Art
In recent years, a technique is widely studied in which a laser annealing is performed on a semiconductor film formed on an insulating substrate of glass or the like to crystallize the film or to improve its crystallinity. Silicon is often used for the semiconductor film.
As compared with a synthetic quartz glass substrate which is conventionally often used, the glass substrate has merits that it is inexpensive and is rich in workability, and a large area substrate can be easily manufactured. This is the reason why the study is carried out. The reason why the laser is used for crystallization by preference is that the melting point of the glass substrate is low. The laser can give high energy only to the semiconductor film without raising the temperature of the substrate very much. Besides, as compared with heating means using an electrothermal furnace, its throughput is remarkably high.
Since a crystalline semiconductor is composed of a number of crystal grains, it is also called a polycrystalline semiconductor film. Since a crystalline semiconductor film formed through the laser annealing has a high mobility, a thin film transistor (TFT) is formed by using this crystalline semiconductor film, and it is extensively used for, for example, a monolithic type liquid crystal electro-optic device in which TFTs for a pixel portion and a driving circuit are formed on one glass substrate.
Besides, a method is used by preference in which a laser annealing is performed by forming a laser beam oscillated from a pulse oscillator with a high output such as an excimer laser into a square spot of several cm square or a line with a length of 10 cm or more on an irradiated surface by an optical system and by scanning the laser beam (or by moving the irradiation position of the laser beam relatively to the surface to be irradiated), since the method has high productivity and is industrially superior.
Especially, when a linear beam is used, differently from the case where a spot-like laser beam requiring horizontal and vertical scanning is used, since laser irradiation on the whole surface to be irradiated can be performed by only scanning in the direction normal to the longitudinal direction of the linear beam, the productivity is high. The scanning is performed in the direction normal to the longitudinal direction since it is the most efficient scanning direction. By this high productivity, in the laser annealing method at present, to use the linear beam obtained by forming a pulse oscillation excimer laser beam through a suitable optical system has become the main stream of manufacturing technique of a liquid crystal display device using TFTs.
Here, crystallization of a semiconductor film after irradiation of a laser beam to the semiconductor film will be described. When the laser beam is irradiated to the semiconductor film, the semiconductor is melted. However, as a time elapses, the temperature of the semiconductor film is lowered, and crystal nuclei are created. In the semiconductor film, a limitless number of uniform (or irregular) crystal nuclei are created and grow, so that crystallization is ended. The positions and sizes of crystal grains obtained in this case are random. As compared with the inside of the crystal grain, at the interface of the crystal grain (crystal grain boundary), there are countless recombination centers and trapping centers resulting from amorphous structure, crystal defects and the like. It is known that if a carrier is trapped by the trapping center, the potential of the crystal grain boundary is raised and becomes a barrier against the carrier, so that the current transport characteristic of the carrier is lowered. Especially although the crystallinity of a semiconductor film of a channel formation region exerts a great influence on the electrical characteristics of a TFT, it has been almost impossible to remove the influence of the crystal grain boundary and to form the channel formation region by a single crystal semiconductor film.
Besides, it is known that the growth distance of the crystal grain is proportional to the product of a crystallization time and a growth speed. Here, the crystallization time is a time, as shown in FIG. 28, from (B) the creation of crystal nuclei in a semiconductor film to (C) the end of crystallization of the semiconductor film. When a time from (A) the start of melting the semiconductor film to (C) the end of the crystallization is called a melting time, if the melting time is prolonged and the cooling speed of the semiconductor film is made slow, the crystallization time becomes long, and a crystal grain of a large grain size can be formed.
Although there are various kinds of laser beams, crystallization using a laser beam from a pulse oscillation type excimer laser as a light source (hereinafter referred to as an excimer laser beam) is generally used. The excimer laser has merits that its output is high, and repetitive irradiation at a high frequency is possible, and further, the excimer laser beam has a merit that an absorption coefficient to a silicon film is high.
In order to form the excimer laser beam, KrF (wavelength of 248 nm) or XeCl (wavelength of 308 nm) is used as an exited gas. However, there is a problem that a gas such as Kr (krypton) or Xe (xenon) is very expensive, and when the frequency of a gas exchange becomes high, manufacturing costs are increased.
Besides, the exchange of additional instruments, such as a laser tube for laser oscillation and a gas purifier for removing unnecessary compounds produced in an oscillation process, becomes necessary once in two to three years. Most of these additional instruments are expensive, and there is a problem that the manufacturing costs are increased as well.
As described above, although the laser irradiation apparatus using the excimer laser beam has undoubtedly high performance, it takes much time and labor in maintenance, and the apparatus has also a defect that running costs (here, costs caused from working) are high for a laser irradiation apparatus for production.
Then, in order to realize a laser irradiation apparatus having low running costs as compared with the excimer laser and to realize a laser annealing method using the same, there is a method using a solid laser (a laser using a crystal rod as an oscillation cavity and for outputting a laser beam).
It is conceivable that the reason is that although the solid laser at present has a large output, an output time is very short. An excitation method of the solid laser includes LD (Laser Diode) excitation, flash lamp excitation, and the like. In order to obtain a large output by the LD excitation, it is necessary to make a large current flow to the LD. Thus, the lifetime of the LD becomes short, and eventually, the cost becomes high as compared with the flash lamp excitation. By such reason, almost all solid lasers by the LD excitation are small output apparatuses, and it is under development as a high output laser for industry at present. On the other hand, since a flash lamp can output extremely intense light, the laser excited by the flash lamp has a high output. However, in the oscillation by the flash lamp excitation, electrons excited by the instantaneously applied energy are emitted at one time, and an output time of the laser becomes very short. Like this, although the solid laser at present has a high output, the output time is very short. Thus, it is difficult to realize formation of a crystal grain, which has a grain size equivalent to or larger than a grain size formed by laser crystallization using the excimer laser, by laser crystallization using the solid laser. Incidentally, in the present specification, the output time is a half value width in one pulse.
Here, crystallization of a semiconductor film was performed by using a YAG laser as one of typical solid lasers. The YAG laser by the flash lamp excitation was used, and after modulation to the second harmonic by a nonlinear optical element, a silicon film was irradiated. The grain size of a crystal grain formed by the laser annealing using the YAG laser was very small as compared with the crystal grain formed by using the excimer laser. When a TFT is manufactured by using a crystalline semiconductor film having such a crystal grain, a large number of crystal grain boundaries exist in a channel formation region having an important influence on the electrical characteristics of the TFT, which cause the electrical characteristics to be lowered. It is conceivable that the reason why only the small crystal grain is formed by the laser annealing using the solid laser is that, as already described, although the solid laser at present has a high output, the output time is very short.